1. Field of the Invention
The present invention relates to a vibration control system for simulating a vibration environment.
2. Prior Art
Mechanical vibration that an apparatus bears in transit or operation causes trouble to the apparatus. In order to examine vibration resistance ability of the apparatus, simulated vibration testing is carried out at the pilot stage and/or mass production stage of the apparatus. In vibration testing, a vibration control system controls a vibration induced on the apparatus as a test object.
The nature of vibration of the field environment is generally random and has no regularity like a sinusoid has. The simulation of such random vibration waveform itself has proved to be of very considerable difficulty. Therefore, in vibration testing, the power spectral density (P.S.D.) of the field vibration is measured and then vibration having a required power spectral density (hereinafter referred to as "reference P.S.D.") is induced on the test object.
Even when the time-domain signal having a reference P.S.D. is applied to a vibration generator, vibration having the reference P.S.D. is not induced on the test object. This is because the vibration generator has an uneven frequency response characteristic. Therefore, a drive spectrum is computed by modifying the reference P.S.D. by an uneven frequency response characteristic of the vibration generator, and a time domain signal having the determined drive spectrum is generated. Then, the time domain signal is applied to the vibration generator. Above mentioned vibration test system has been disclosed in U.S. Pat. No. 3,710,082 or "Random vibration test system using digital equalizer" by Alfred G. Ratz; Proceedings of 1970 Annual Technical Meeting of the Institute of Environmental Sciences, Apr. 1970.
In such a system, vibration of test object is detected and converted into a digital signal. This digital signal is converted into the frequency-domain by the Fourier transform and the spectrum data are averaged to estimate a response P.S.D. The response P.S.D. is compared with the reference P.S.D. and the drive spectrum is computed. The drive spectrum is converted into a time domain signal (the drive signal) by the inverse Fourier transform. The drive signal is converted into an analog signal and then is applied to the vibration generator on which the test object is fixed.
Usually a batch of digital signal data of N points (corresponding to T sec) is handled at a time in the procedure (hereinafter we call this N points of data sequence "one frame"). If the drive signal of one frame should be generated corresponding to the detected response signal of one frame, very high speed computing is necessary and the system must be expensive. Therefore, some buffering mechanism is required in the data processing procedure.
For such a purpose, U.S. Pat. No. 3,848,115 discloses a vibration control system wherein plural frames of the drive signal are obtained by a randomization process from a single frame of the time domain signal which is computed from the drive spectrum by the inverse Fourier transform. However, because the plural frames of the drive signal are generated from a single frame signal, each obtained frame of plural frames of the drive signal has not enough statistical independence.